Sophisticated analytical techniques of the present day include the use of various modes of spectroscopy that involve excitation of a sample and subsequent observation and/or quantification or the resultant changes in energy as manifested by spectral emissions. These modifications to the energy of the sample can be induced using focused electro-magnetic energy emissions at a predetermined wavelength, or even using electro-magnetic radiation. Thus, a spectrograph typically includes an external source of electromagnetic energy and means to disperse the energy into its energy components. A detection system is used to measure and/or record the dispersed energy components. The current invention is helpful in allowing the observation of such emissions as it reduces the amount of incident electromagnetic energy onto the detection means by rejecting the undesirable non-synchronous background electromagnetic energy otherwise known as noise and/or background interference. This effectively increases the relative strength of the desired signal or spectrum.
The present invention more precisely relates to the use of an extremely weak light signal from pulsed optical techniques, such as spontaneous Raman scattering (SRS), to measure gas-phase molecular densities and temperatures in combustion environments. However, this use in particular often suffers from large sources of background interference such as flame and soot luminosity that are substantially brighter than the faint amounts of signals derived form the weak Raman effect. Time gating the amount of light reaching the detector can improve the signal to noise ratio (SNR) for techniques such as SRS, and laser induced fluorescence (LIF) and other low energy laser driven techniques.
The prior art has recognized that damping background light can alleviate this problem. One approach presented in U.S. Pat. No. 4,956,897 is to use a liquid crystal array along with Hadamard encodement as a stationary electro-optical mask. Other prior art techniques utilize liquid crystal display (“LCD”) shutters without the Hadamard mask. Further, a photo-multiplier tube has been used as a gated integrator for a single channel detector in conjunction with a spectrograph. Image intensifiers have also been employed as an optical gate for use with pulsed laser excitation sources for multi-channel detection in spectrograph. Problems encountered with some of these prior art techniques include low quantum efficiency, decreased signal to noise ratio (SNR), and a low dynamic range of about 102, or 103 at best. In order to provide higher quantum efficiency, higher SNR and more dynamic range, it has been proposed to use non-image intensified charge-coupled devices (CCD's) for the detection of weak pulsed light signals. However, non-intensified CCD's do not have the ability to gate-out background light interference on the time scale of the laser pulse resulting in large background signal levels. Typically, large format leaf shutters are used with non-intensified CCD arrays to reduce levels of background. However, typical test shutters have minimum gate widths of about 10 ms. In a 10 ms time period, substantial levels of background light overwhelms the signal from the Raman effect which is in the range of 100 to 10,000 photo-electrons for a typical SRS measurement.
Another prior art response to these problems have included attempts to gate non-intensified CCD's using ferroelectric light controllers (FLC), and have provided gate widths as short as 35 μs (microseconds), but with a relatively low (i.e. 25%) optical transmission efficiency in the on-state and typically a 500:1 contrast ratio. The use of FLC's however, was only marginally better than the use of intensified CCD's due to the FLC's reduced optical efficiency and wider temporal gate width. Other techniques in the prior art include the use of slit scanning optics to limit the transmission and to block incidental background light. One such technique is set forth in U.S. Pat. No. 5,457,530 which utilizes an optical shutter array member having a slit plate made of a piezoelectric material such as PZT (lead zirconium titanate). Another technique using a rotary chopper with a mechanically geared motor was developed by R. S. Barlow and P. C. Miles, “A shutter-based line imaging system for single-shot Raman scattering measurements of gradients in mixture fraction”, Proceedings of the 28th International Symposium on Combustion, Edinburgh, Scotland (2000). This system utilized geared high-speed (21,000 rpm) rotary chopper blades. However, these prior art techniques suffer from being complex and difficult to build and maintain at the desired speed of operation.
In addition, the current invention is useful for devices that produce short pulses of light from continuous wave light sources such as a stroboscope or a high repetition rate laser source. It is useful as a fiber optic light gate at microsecond speeds. It is also useful as a high-speed molecular beam shutter used in high-vacuum systems.